What advice would you give to someone going to Korea (first duty station) who has never been out of the country before?

Do not drink anything with ice and brush your teeth with bottled water. the water over there will make you wish it had killed you

Sir...You have asked an excellent question...so I'm going to add my "2 cents worth".
First: ALWAYS REMEMBER...NEVER FORGET...You area quest in this country. You will be subject to their laws. You screw up...and you WILL "pay the price".
You represent the USA... DON'T be arrogant or haughty. You have been given an opportunity to visit a
place that many can just dream about. Set some "travel books", take pictures, learn a little of the language (forget about reading...man...that's confusing!).
Check with your base's Services Center. They'll help you a lot.
You will see many things on the "econ" that are different from us and "our ways". Ask why...to learn,
don't be judgemental, remember where you're at. Buy and ship home your souvenirs and mementos. They're great to add to your stories when you get back home. They'll bring back the memories.
STAY AWAY FROM DRUG USAGE. You'll just get the "short end of the stick", you just and might be "singled out" by LEO or the MP's... or CID.
Hook up with a friend and explore together...Meet a "local lady" to help teach and have a good time with, not a "Bar Fly" that only wants your bucks. And even then...show them respect, you'll get respect back.
All in all...do your job. When you're off duty...HAVE SOME FUN! Don't hide "on base".
As an "old MP" stationed in Japan: '70 -'72, these things I learned...some easy...some not so easy. I had to enforce the law and deal with those who didn't follow it...it's not that difficult...really.
But if you stay with your common sense, don't "run with the crowd" only, "be casual", and don't "overspend", you will have memories and stories to share...forever.
That's my "shpeal"..."the end".

Tell them no when your "mates" suggest a green bean tour.

DONT GET MARRIED TO THE FIRST GIRL YOU DATE!!!

1. Don't get a haircut just after lunch from your Korean barber. The kimchi breath will knock you out.
2. Don't fall for the first moose who thinks you're adorable. Always use protection in any case.
3. Do watch for quicky pickpockets and watch snatchers. They are good at what they do.
4. Do get your uniforms and equipment displays IAW with company policies. The Korean houseboy can help make it happen.
5. Do be quick to get back to the compound before midnight. Consequences are tough.
6. Don't get involved with the black market that abounds everywhere. Heavy penalties are there.
7. Take guard duties seriously.
8. Get to know some decent Korean families and enjoy their customs and traditions.
9. Learn and sing the words to "Arirang," the now unofficial National Anthem of Korea.
10. Let your houseboy teach you the Korean language if you can do it. Learn decent phrases.
And last, enjoy your tour while you grow accustomed to the smell of honey wagons.

I did two tours in Korea: 1. 304th Sig Bn in Seoul (1962) and 2. SSO 2ID, Tong du Chon (1975)

Get out and see the countryside! Don’t stay walled-up in your barracks... It might be a once in a lifetime experience to see a country you might not ever see again, or, you might see a number of times!
Watch yourself with the alcohol as it WILL get you into trouble! It is really a good tour to get ahead of your peers for schooling while they might be partying... Study for SM boards go to as many as you can, good practice!
Have fun, but be smart about your single life, disease central if you aren’t careful for your little unarmed, unprotected member, again, be S-M-A-R-T!!!
About all I can say as I haven’t been in 20 yrs. Good luck troop!

The advice you have already been given is wise and I hope you listen. My additional comment are:
It doesn't matter what rank you are - you represent the image and reputation of the United States, the US Army and most import: Yourself! Don't tarnish it. Work hard, get promoted and be thankful when you return state side to having been a part of history. There are definitely far worse assignment and many far better assignments. But at this time in our history, your service in Korea may either see major milestones of improvement for the USA & N. Korea relations or potentially the opposite.

Learn their customs, some things we consider a norm in America may be an insult or disrespectful in another country. Example in America we rub our eyes which to us it just means someone has tired eyes or something in them,in Australia it means down with the queen. In some countries its an insult to point the sole of your foot towards a person, it means you are lower than the sole of my feet, as in crossing your legs.

Pay attention to the recall times. If you are single and can't get back on base before the recall time, stay in a motel or go into one of the 24 hours saunas. Just get off the streets during recall time.
Juicy girls love your money, not you.
See the sights of the country. Travel. But don't act the fool. You represent the US whether you are in uniform or not.

Get off base. If someone says "there is nothing to do" avoid them, they are clueless. If you are bored you aren't half trying. Take MWR tours, do day trips when possible. Remember that people pay thousands of dollars to go on vacation where you will be living and working for a couple of years.